It is oftentimes desirable to remotely monitor particular locations, including objects or live subjects thereat, to determine or ascertain a condition or a change in condition. Such might be the case, for example, in monitoring a location for the presence of a condition such as fire, high or low temperature, radiation, and/or light or the condition of a live subject to determine a change in condition such as in temperature, heart rate, or pulse rate.
The desirability of monitoring buildings and the like for early detection of abnormal and/or unwanted conditions such as a fire is well known and various devices and/or systems have heretofore been suggested and/or utilized to provide for such detection. In addition, some such systems have provided information as to the location of the detected condition.
Where a single or only a few detector stations are necessary, the problem of detecting the location of a condition such as a sensed fire is not overly troublesome, but when the number of stations is increased, the problem becomes more difficult, and has heretofore resulted either in complicated and expensive systems, complicated and expensive wiring between stations, and/or in systems that could not adequately handle the problem.
Alarm monitoring systems with remote detectors are shown, for example, in U.S. Pat. Nos. 3,189,882; 3,553,730; 3,566,399 and 3,735,396. Alarm monitoring systems utilizing time delayed responses are shown, by way of example, in U.S. Pat. Nos. 3,233,232 and 3,508,260.
Alarm monitoring systems utilizing variable pulse widths to trigger an alarm condition are shown, again by way of example, in U.S. Pat. Nos. 3,293,605 and 3,392,374. Other examples of signals utilized for indicating an alarm condition include utilizing the absence of a signal (U.S. Pat. No. 3,426,348); using voltage or current differentials (U.S. Pat. Nos. 3,676,877; 4,032,907; 4,041,455 and 4,095,220); using frequency differentials (U.S. Pat. No. 3,886,534); using a combination of synchronization and energizing pulses (U.S. Pat. No. 4,030,095); using pulse duration modulated signals (U.S. Pat. No. 3,940,739); using pulse position modulation (U.S. Pat. Nos. 3,689,888 and 3,940,739); and using pulses for sequential interrogation of remote transmitters (U.S. Pat. Nos. 3,278,920; 3,611,361 and 4,067,008).